Protoplaneter

Protoplaneter is a term used to denote a nascent planetary body in the early stages of planet formation within a protoplanetary disk around a young star. In this usage, a protoplaneter is essentially synonymous with a protoplanet or planetary embryo, representing the intermediate phase between a planetesimal and a fully assembled planet. The term is not widely used in scientific literature; most researchers prefer protoplanet or planetary embryo to describe such objects.

Formation and growth occur by accretion of solids and gas. In the core accretion model, a solid core grows until runaway gas accretion can begin, while pebble accretion can accelerate core growth by efficiently accumulating small solids. The location within the disk and the local density of solids influence growth rates. As a body becomes massive enough, it can interact gravitationally with the disk, leading to orbital migration (Type I for lower mass, Type II for gap-opening masses) and sometimes to resonance capture with other bodies. Protoplaneters may also clear gaps or rings in the disk as they accrete and migrate.

Observational evidence for such objects is indirect in most cases. Gaps and rings observed in protoplanetary disks with facilities like ALMA are interpreted as signatures of embedded forming planets, and a few direct detections of candidate forming planets have been reported. However, direct confirmation remains challenging, and the term protoplaneter is less common than protoplanet or planetary embryo in the literature.

Related concepts include protoplanet, protoplanetary disk, planet formation, and planetary embryo.